Information recording media such as CDs, CD-ROMs, CD-Rs, DVDs, PDs, MOs, and MDs have hitherto been widely used as massive recording medium that store sound or image signals. In particular, much attention has been paid to photomagnetic recording media on which at least information recording is carried out when the media are irradiated with light and receive applied magnetic fields. This is because these media are high-density recording media that enable rewriting of information. Much effort has been made to research and develop these recording media in order to further increase the recording density. Further, photomagnetic recording apparatuses have also been researched and developed in order to enable information to be reproduced from or recorded on such photomagnetic information recording media at higher speed.
The conventional photomagnetic information recording apparatus employs a light modulating system that records information on a recording medium by light modulation in accordance with the information. However, with the increased recording density, there is a tendency to employ, instead of the conventional light modulating system, a magnetic modulating system that records information by modulation of magnetic fields in accordance with the information.
A photomagnetic information recording apparatus using the magnetic field modulating system concentrates laser light for recording to make the temperature of a recording film of a recording medium close to a Curie point. Then, in this state, the apparatus applies magnetic fields generated by a coil to the recording film to adjust the magnetizing direction of the recording film in accordance with the information. The apparatus thus records the information.
To use such a photomagnetic information recording apparatus using the magnetic field modulating system to record or reproduce massive data at high speed, it is desirable to have a front illumination type configuration in which an optical system that concentrates light on a recording medium and a coil that generates magnetic fields are arranged on the same side, as viewed from the recording medium. In this configuration, in general, the optical system is placed on one surface of a glass substrate, while a spiral magnetic coil is placed on the other surface. To use the magnetic field modulating system to record and reproduce at high speed, it is necessary to switch the direction of magnetic fields applied to the recording film at high frequency. The above configuration provides a small-sized low-inductance magnetic field coil that requires reduced power. It is thus possible to provide a magnetic field coil that can be driven at high speed.
FIG. 1 is a schematic diagram showing the structure of a common magnetic field generator of a front illumination type.
In a magnetic field generator 7 in FIG. 1, an optical lens 72 is placed on a top surface of a glass substrate 71. A dielectric layer 73 is provided on a bottom surface of the glass substrate 71 which is opposite the top surface. A lens 8 is provided above the magnetic field generator 7 to concentrate laser light L on the optical lens 72. The laser light L concentrated by the lens 8 is further concentrated by the optical lens 72, provided on the top surface of the glass substrate 71. The resulting laser light L passes through the glass substrate 71 and dielectric layer 73. A recording layer 91 of a magnetic recording medium 9 is then irradiated with the laser light L. A coil is placed in the dielectric layer 73. The coil extends spirally in a direction in which the dielectric layer 73 extends, so as to surround an area through which the laser light L passes. Such a magnetic field coil composed of a thin film can be produced using a semiconductor process.
When a current is passed through the coil, the coil generates heat. In general, the electric resistance of a substance increases with increasing temperature. Thus, if the heat generated by the coil is not efficiently radiated, the coil lapses into a vicious circle in which it consumes more power owing to its own heat generation, thus further increasing the quantity of heat. The dielectric layer, in which the coil is provided, does not have a high thermal conductivity. Accordingly, the heat generated by the coil is not readily radiated. As a result, the temperature of the coil may increase rapidly to damage the coil before it can generate magnetic fields of a predetermined intensity. It is therefore an important object to radiate the heat generated by the coil.
It is thus possible to place a metal such as copper which has a high thermal conductivity, around the periphery of the coil so that the heat generated by the coil can be transmitted through the coil to be radiated to the exterior.
FIG. 2 is a diagram showing how a metal having a high thermal conductivity is placed around the periphery of a coil. FIG. 3 is a diagram of the coil shown in FIG. 2 and through which a current is passed, as viewed from a photomagnetic recording medium.
In these figures, the same components as those described above will be denoted by the same reference numerals. A coil 74, a yoke 75, and a radiator 76 are arranged inside the dielectric layer 73, provided in the glass substrate 71 shown in FIG. 2, the dielectric layer 73 made of alumina. The coil 74 extends spirally so as to surround an area through which the laser light L passes. The radiator 76 is a nonmagnetic metal film that extends so as to surround the periphery of the coil 74. The yoke 75 is a magnetic film that extends between the coil 74 and the glass substrate 71 so as to cover the coil 74 and a part of the radiator 75 which is closer to the coil 74. The yoke 75 functions as a core of the coil and also as a radiation path for heat generated by the coil 74 because it has a higher thermal conductivity than the dielectric layer 73. The heat generated by the coil 74, shown in FIG. 2, passes through the yoke 75 and is then radiated to the exterior from the radiator 76.
When a high-frequency current is passed through the coil 74, as the current flowing through the coil increases, an induced current (eddy current) (see arrow Ie in FIG. 3) flows through the radiator 76, which is a conductor, the induced current flowing in a direction opposite to that of the current flowing through the coil 74 (see arrow I in FIG. 3). This may disadvantageously weaken magnetic fields generated by the coil 74.
To radiate the heat generated by the coil while solving the above problem, it is possible to employ a technique for distributively arranging small copper pieces so that the pieces surround the periphery of the coil 74 (see, for example, Patent Documents 1 and 2. However, since each of the copper pieces is very small, a high radiation efficiency is not expected. Accordingly, the techniques described in these patent documents cannot efficiently radiate the heat generated by the coil.
(Patent Document 1)
Japanese Patent Laid-Open No. 10-255207
(Patent Document 2)
Japanese Patent Laid-Open No. 11-316901